Precision casting mold and method of making the same



March 31, 1959 H. SCHNEIDER PRECISION: CASTING MOLD AND METHOD? OF"MAKING-.THE SAME Filed Dec. 14., 1954 lllndllllldlfllllallirrflllllllllflllllllllllll x ,frail JNVENToR. Hons SchneiderFfa@ @Ml/nw MTV@ ATTORNEYS The pattern assembly as shown in Fig. 2 isready for coating by immersing or spraying with a mixture consisting ofa solid filler material and a suitable liquid binder. The binderserves'to provide a close refractory film and the refractory fillermaterial should have the property of forming a very smooth andrefractory surface in contact with the wax pattern so that the castarticle will be as smooth as possible.

The pattern assembly is then placed on a molding ask base plate 14provided with a hole or perforation 1S as shown in Figs. 2 and 3. Acylindrical flask'16 is then fitted over the whole pattern assembly andthe joint between base plate 14 and the'flask 16 sealed with wax or byany other suitable sealing composition. A quick setting refractorymixture is thenpo-ured into the flask thus formed and permitted to standuntil the mixture has suiciently solidified to form a plate 19.Thereafter the dry, pourable, backing material is filled into the ask.The backing material consists of a refractory composition, a binder, orbonding agent, and a carrier for the bonding agent. The refractorymaterial may consist of a granular material such as molding sand orquartz. The binding material is composed of a dry, finely ground powderat room temperature which becomes an effective bonding agent at thetemperatures used to bake the mold. The carrier for the bonding agent isa pulverulent substance having a large active surface and which willreadily absorb the inorganic bonding agent. Advantageously, thepulverulent carrier substance is impregnated with a solution of thebonding agent or with a fluid medium containing the bonding agent in afairly finely divided solid form and subsequently dried. The driedpulverulent mass is then mixed with the refractory component of theinvestment composition. An advantage of using such a carrier for thebonding agent is that it brings a very large total surface area of thegranular refractory molding compound into contact with the bonding agentadhering tothe surface or pores of the pulverulent carrier.

The pattern assembly provided with the thin non-selfsupporting coating17 is thus completely embedded in the backing material 18. The wholeassembly is then placed on a vibration table, and vibrated sufficientlyto insure that all surfaces of the coated pattern assembly are backedbythe backing material. Thereafter a cover 20 is formed by pouring thesame refractory mixture as used for the plate 19 on to the surface ofthe backing material.

In the next operation, the assembly shown in Fig. 3 is transferred intoa furnace and fired by heating at a temperature of approximately 80G-900C. for 6-8 hours. This lfiring treatment causes the pattern material andthe sprue material to melt. The wax or synthetic resin will partly flowout through the hole or perforation 15 or be burned. On baking the moldthe bonding agent imparts a uniform cohesive strength to the investment.The investment compound having thus been consolidated has the propertyof protecting the thin mold layer effectively against destruction by theinowing metal.

Fig. 4 shows the fired mold ready for casting. Molten metal is pouredthrough the ingate 21 While the mold is still hot. The ring of the moldtransforms the backing material into a refractory body 22 to which thecoating 17 adheres.

Various granular, dry, refractory materials such as molding sands orquartz may be utilized as the refractory constituent of the backingcomposition Without departing from the scope of the present invention.

The utilization of the dry pourable backing composition of the presentinvention entails a particularly careful bonding of the granular,refractory constituents of the backing up composition which on bakingthe mold im- Vately high temperature.

4 a high adhesivity or a great capacity of sticking to the granularrefractory compound of the backing up composition. Particularly suitableare gel-like inorganic compounds pulverulent at room temperature andwhich become fusible at temperatures above 600 C. and preferable between800 and l000 C. The backing up composition, having been bonded, isadapted to protect the thin mold layer reliably against destruction bythe intlowing molten metal. l

Some examples of suitable inorganic bonding compounds are liquiddispersions of gelatinous bonding agents such as complex aluminiumpyrophosphates, alkali-metal Zirconates, alkali-metal titanates, titanicacids, zirconic acids, or oxychlorides such as magnesium oxychlorides.These compounds are distinguished in that at room temperature theyexhibit a gel-like structure in a dry condition. Other examples aresolutions of borates, such as borax dissolved in water, phosphoric acidand boric acid. Solutions of phosphates such as primary sodium phosphatemay also be used.

lVarious carriers for the inorganic bonding agents may be used inaccordance with the present invention. Some examples of carriers whichhave large active surfaces and exhibit a high adhesivity or capacity ofabsorbing the inorganic bonding agents due to their porous constitutionand their highly particulate surface are infusorial earth, magnesiumsilicates such as meerschaum, pumice, and asbestos. Other substanceshave a large active surface and the above properties may also be used asa carrier in accordance with the present invention. The above namedcompositions, however have been found to be particularly advantageous inthe present invention.

When larger size patterns are to be invested'or backed it is alsoadvantageous to admix with the backing up composition comprising a dry,pulverulent, refractory material, an inorganic bonding agent effectiveat ternperatures above 600 C. and a carrier for the inorganic bondingagent, a second bonding agent dry at room. temperature and becoming aneffective bonding agent at temperatures below the baking temperatures ofthe mold, or below 600 C. Examples of such bonding'agents are nelyground synthetic resins, such as silicone resins, which become aneffective bonding agent at a moder- This second bonding agent may beused when the rigidity of the raw unbaked molds do not resist thevibrations occurring during their transportation.

Example A plurality of wax patterns of a small flat bottomed hollowcylindrical article were assembled together with a sprue in the form ofa tree-like structure which serve as a positive pattern fortherproduction of the casting mold. The sprue was similarly -made ofwax. The composite Vtree-like pattern thus constructed was then dippedinto a liquid mold forming composition of a mixture prepared from finelycomminuted quartz and partly hydrolized yethyl silicate dissolved inalcohol as is well known in the yart. The thin non-self-supporting filmdeposited on the pattern was about 0.5-1 mm. in thickness. This coatingwas then dried and the pattern placed with the sprue downward upon aplate perforated for the subsequent discharge of the molten wax pattern.A cylindrical molders flask made of heat-resisting sheet metal was thenplaced over the entire coated pattern and the joint between the flaskand the perforated plate was then sealed olf with wax. A small amount ofa liquid mixture of quartz sand and water' glass to which an acid, suchas hydrochloric acid, had been added for the purpose of accelerating thehydrolysis, was then added to the ask. This fluid mixture was permittedto stand until solidified to form a cover plate surrounding the sprue.The molders flask was then filled up witha dry, pourabley backing upcomposition which was prepared asy follows: 600 gramsof primary sodiumphosphate was dissolved in v2 liters of Water and the solution added to2.4 kg. of comminuted infusorial earth. Infusorial earth consists of theminute fragments of the shells of silicious algae (diatomae). It ischaracterized by a very large active surface. The mixture of primarysodium phosphate and infusorial earth was thoroughly kneaded and driedovernight at about 120 C. The dried composition was then ground andparticles above 40 microns removed therefrom by means of a ne meshscreen. 6 parts by weight of the pulverulent substance thus obtainedwere mixed with 94 parts by weight of quartz sand having a particle sizebetween 0.5 and 1.0 mm., and the whole thoroughly commingled in a rotarydrum. This dry, pourable backing up composition thus obtained,containing an inorganic bonding agent, was packed into the molders flaskand iirmly settled therein on a shaking table. The top face of themolders tiask was then covered with a slurrylike mixture of quartz sandand water glass, to which an acid had been added to accelerate thehydrolysis, to form a second cover plate the same as that formed above.After the cover had hardened the molders flask was transferred into abaking furnace and gradually raised to a baking temperature of 800 C.during a period of 6-8 hours. During the baking operation the sodiumphosphate deposited upon the infusorial earth fuses and in that stateexerts the desired bonding action on the investment composition. If thewax pattern is to be recovered, the mold prior to being baked, istransferred into a drying oven in which the wax is caused to run out ata slightly raised temperature. Immediately upon removal of the moldersask from the baking furnace, liquid metal was poured in the mold. Aftercooling the mold, the two 'covers were broken up and the waste moldforming cornposition knocked out of the molders ask. The castings may bereadily taken out of the backing up composition and freed from any moldforming composition still adhering thereto in any conventional manner.The finished castings exhibited a perfectly smooth surface free fromblemishes and form a particularly true replica of the pattern parts.

In case of larger size molds, it is advantageous to use armoring membersfor the purpose of strengthening the investment compound. Casting moldsproduced in accordance with the process of the invention are adapted forstatic castings as well as for making castings on a centrifugal castingmachine.

Iclaim:

1. The method of making a unitary precision casting mold by means of apattern destroyable in the mold which comprises coating the pattern withat least one mold forming layer having the form of a thinnon-selfsupporting coating, placing the pattern in a molders llask,backing up the coated pattern with an investment composition comprisinga dry pourable composition containing a granular, refractory, moldingcomposition, an inorganic bonding agent, and a carrier which is a memberselected from the group consisting of infusorial earth, magnesiumsilicate, meerschaum, pumice, and asbestos impregnated with the bondingagent comprising a dry,

pulverulent composition having a large active and porous surface and ahigh capacity for absorbing inorganic bonding agents, the particle sizeof said refractory composition being substantially larger than theparticle size of said impregnated carrier, and heating the mold to thebaking temperature of the mold to destroy the mold pattern and to permitadherence of the pattern coating to the backing up composition, saidcarrier impregnated with said inorganic bonding agent being dry andpulverulent at room temperature and said bonding agent becoming aneffective bonding agent at the temperature used to bake the mold.

2. The method of claim 1 in which the carrier is impregnated with acomposition selected from a member of the group consisting ofphosphates, phosphoric acid, borates, boric acid, complex aluminiumpyrophosphates, zirconic acid, alkali-metal zirconates, titanic acid,and

alkali-metal titanates.

3. An investment back up composition for unitary precision casting moldsin which the mold pattern is defined by at least one mold forming layerin the form of a thin non-self-supporting coating and in which the moldsare formed by baking the coated pattern and back up compositioncomprising a dry, pulverulent, pourable composition containing agranular, refractory, molding composition and a carrier which is amember selected from the group consisting of infusorial earth, magnesiumsilicate, meerschaum, pumice, and asbestos impregnated with an inorganicbonding agent, said carrier comprising a dry, pulverulent compositionhaving a large active a'nd porous surface and a high capacity forabsorbing inorganic bonding agents, and said bonding agent being dry andpulverulent at room temperatures and becoming an effective bonding agentat the temperature used to bake the mold, said back up composition.comprising between about to 96 parts by weight of refractory material,between about 3 to 10 parts by weight carrier, and between about 1 to 5parts by weight of inorganic binder on a dry basis.

4. The investment, back up composition of claim 3 in which the carriercomposition is impregnated with a composition selected from the groupconsisting of phosphates, borates, phosphoric acid, complex aluminiumpyrophosphates, zirconic acid, alkali-metal zirconates, titanic acid,and alkali-metal titanates.

References Cited in the tle of this patent UNITED STATES PATENTS2,027,932 Ray Ian. 14, 1936 2,388,299 'I'hielemann Nov. 6, 19452,441,695 Feagin et al May 18, 1948 2,521,614 Valyi Sept. 5, 19502,682,692 Kohl July 6, 1954 2,720,687 Shaw Oct. 18, 1955 2,736,077Bartlett Feb. 28, 1956 FOREIGN PATENTS 585,665 Great Britain Feb. 18,1947 703,607 Great Britain Feb. 3, 1954

